When a change in luminance value of an image is gradual at, for example, an edge part where a change in luminance value should normally be clear, the image creates a fuzzy impression. A clear image can be obtained by enhancing the change in luminance value of such an image. In a television receiver, a digital camera, and the like, a method for sharpening is adopted for improving the clearness of an image.
In this method, an adjustment of color-difference signals is generally performed so as to keep the ratio of luminance signals to color-difference signals that existed before the adjustment. That is to say, levels of luminance signals and color-difference signals are corrected so as to keep the ratio (the Y/C ratio) of luminance signals (Y) to color-difference signals (C) of original colors (so as to keep the Y/C ratio constant). Such an adjustment of color-difference signals is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-252584.
However, in correction for keeping the Y/C ratio constant (correction of color-difference signals), when levels of luminance signals (Y levels) are decreased by gradation conversion, an image appears dark in some practical cases. Moreover, when levels of color-difference signals (C levels) are large, levels of luminance signals (Y levels) are small, and the levels of luminance signals (the Y levels) are increased by gradation conversion, the levels of color-difference signals (C levels) are also increased to keep the Y/C ratio constant. Thus, the levels of color-difference signals (C levels) disadvantageously reach a saturation point.